Molten-air battery

Researchers at George Washington University, led by Stuart Licht, have demonstrated a new class of high-energy battery, called a "molten-air battery," that has one of the highest storage capacities of any battery type to date. Unlike some other high-energy batteries, the molten-air battery has the advantage of being rechargeable.

Experiments

The researchers started with iron, carbon or vanadium boride for their ability to transfer multiple electrons.

Molten air batteries made with iron, carbon or vanadium boride can store three, four and 11 electrons per molecule respectively, giving them 20 to 50 times the storage capacity of a lithium-ion battery, which is only able to store one electron per molecule of lithium. "Molten air introduces an entirely new class of batteries," Licht said.

Other multiple-electron-per-molecule batteries the Licht group has introduced, such as the super-iron or coated vanadium boride air battery, also have high storage capacities. But they had one serious drawback: They were not rechargeable.

Here we show three examples of the new battery's electron transfer chemistry. These are the metal, carbon and VB2 molten air batteries with respective intrinsic volumetric energy capacities of 10000 (for Fe to Fe(III)), 19000 (C to CO32−) and 27000 W h l−1 (VB2 to B2O3 + V2O5), compared to 6200 W h l−1 for the lithium air battery. Higher energy capacity, cost effective batteries are needed for a range of electronic, transportation and greenhouse gas reduction power generation devices. Needed greenhouse gas battery reduction applications include overcoming the battery driven “range anxiety” of electric vehicles, and increased capacity energy storage for the electric grid.

How these batteries work?

As the name implies, air acts as one of the battery electrodes, while simple nickel or iron electrodes can serve as the other. "Molten" refers to the electrolyte, which is mixed with reactants for iron, carbon or vanadium boride, then heated until the mixture becomes liquid. The liquid electrolyte covers the metal electrode and is also exposed to the air electrode.

Where are innovations?

Well, we know that there have been rechargeable batteries that use molten electrolytes before, so where're innovations and why people haven't used these batteries before?

It's the tricky question.There have been rechargeable batteries that use molten electrolytes,but without air.

For example, molten-sulfur batteries have been widely studied for electric car and grid applications. However, sulfur is twice as massive as oxygen (per electron stored) and its mass needs to be carried as part of the battery (whereas air is freely available).

In the end we received a batteries with a low storage capacity, that are not appropriate for EVs at all.

The molten-air batteries are the first rechargeable batteries to use a molten salt to store energy using 'free' oxygen from the air and multi-electron storage molecules."

This ability to store multiple electrons in a single molecule is one of the biggest advantages of the molten-air battery.

Recharging process

It wasn't easy to made battery rechargable and researchers explain that they were able to make the battery reversible by using an unusual electrolytic splitting process(simply by reinserting a large number of electrons) to function as battery "charging." For example, when the iron molten-air battery is discharged, the iron mixes with the oxygen to produce iron oxide. To charge the battery, the iron oxide is converted back into iron metal, and O2 is released into the air(rechargeable battery uses oxygen directly from the air, not stored, to yield high battery capacity).

The carbon and VB2 molten-air batteries recharge in a similar way, although the electrochemical properties of VB2 are not as well understood as the others.

As Licht explained, the molten electrolyte is a key to making the battery rechargeable.

"In the case of molten-air batteries, the molten electrolyte opens a pathway to recharge a wide variety of high-capacity multi-electron storage materials," he said. "These materials, while highest in capacity, are a challenge to recharge (how do you reinsert 11 electrons back into each molecule of vanadium boride?).

The molten electrolyte provides an effective media that is compatible with both recharging these materials and 'free' oxygen from the air for storage. The high activity of molten electrolytes allows this charging to occur."

High temperature requirement

The electrolytes are all melted to a liquid by temperatures between 700 and 800 degrees Celsius and such high-temperature requirement is challenging to operate inside a vehicle, but we also know that such temperatures are reached in conventional internal combustion engines and scientists are still working under this problem, so it's not that big of a deal.

In progress

While the molten-air battery's high capacity and reversibility make it an attractive candidate for future energy storage applications, the researchers are continuing to improve other areas of the battery. For example, they investigate other types of molten electrolytes with lower melting temperatures, increasing the voltage (a major contributor to power density and, for electric vehicles, maximum speed), and improving the energy efficiency.

"It's really important to make the batteries viable candidates for extending electric cars' driving range. In the Licht group's latest study, the molten air battery operating temperature has been lowered to 600 degrees Celsius or less.

The new class of molten-air batteries could also be used for large-scale energy storage for electric grids. "A high-temperature battery is unusual for a vehicle, but we know it has feasibility," Licht said. "It presents an interesting engineering question."